Circular cylindrical metal shells are widely used in large civil engineering projects as, for example, penstocks in dam projects, blast tubes and in the fabrication of high-pressure structures, including vessels.
One type of high-pressure vessel which is suitably employed in many industrial processes has a circular cylindrical shell body, which is generally positioned horizontally or vertically, and end closures which can be hemispherical, elliptical or conical shells or even be flat ends. Many pressure vessels of the described shape are shop fabricated and then transported to the site for erection. However, the large size and weight of some such high-pressure vessels prohibits shop fabrication so field fabrication at the site is necessary.
Whether the high-pressure structure, such as a vessel, is shop or field fabricated, the cylindrical shell is generally made from metal rings which are joined together in consecutive order by weldments which connect abutting edges of adjacent rings. Because the metal rings often are made of metal plates 3 to 12 inches or more thick, and have a diameter of 3 to 30 feet or more, the weldments used to join the rings are time consuming and costly to make. Furthermore, 8 inch thick rings are generally the maximum joined together in field or site fabrication. While vessels greater than 8" thick can be site erected, vessel construction gets more complicated and therefore more costly for vessels greater than 8" thick. Some of the complications are:
A. Eight inches (8") is a practical limit for isotope radiation sources for radiographic examination of weldments. Therefore, when the welds are thicker than 8", a linear accelerator must be used for radiography which will require extensive shielding of the area during radiography. This will complicate the completion of these welds making the costs higher and requiring longer schedules.
B. SA-387 Grade 22 Class 2 steel is one of the commonly referenced vessel materials used in the petroleum refining industry. This material requires post weld heat treatment (PWHT) hold times which are a function of weld thickness (Table AF-402.1 of ASME Section VIII, Division 2). As the material gets thicker more PWHT time is required and it is more difficult to provide material that can withstand these long PWHT times and still maintain the specified strength properties after PWHT. Therefore, for very thick walled vessels, the materials are more expensive per unit weight and are also available from fewer sources.
C. For large site-erected heavy-wall vessels, individual cylindrical sections of the shell are assembled on the ground or at a shop manufacturing facility and then shipped to the site. These rings are then lifted into place and the girth seams welded together. The time required to complete these girth seam weldments has a very direct relationship to the construction schedule for heavy-wall vessels. The time and labor to complete the girth seam weldment increases as a quadratic function of the weld thickness. The overall cost and schedule for constructing very thick walled vessels has a negative impact on overall plant costs.
Much of the above discussion also applies to the fabrication of metal penstocks, blast tubes and similar large size structures having metal shells which are open-ended.
From the above it is clear that a need exists for improved circular cylindrical metal shells which can be used in high-pressure structures, including penstocks, blast tubes and similar open-ended shells, and also as part of thick-walled high-pressure vessels, which can be shop and field fabricated with lower costs and in a shorter time than has been previously possible.